grape pigment (malvidin-3-fructoside) as natural sensitizer …...grape pigment...

7
ORIGINAL PAPER Grape pigment (malvidin-3-fructoside) as natural sensitizer for dye-sensitized solar cells N. Gokilamani N. Muthukumarasamy M. Thambidurai A. Ranjitha Dhayalan Velauthapillai Received: 18 January 2014 / Accepted: 24 May 2014 / Published online: 6 July 2014 Ó The Author(s) 2014. This article is published with open access at Springerlink.com Abstract TiO 2 nanocrystalline thin films have various applications, among them dye-sensitized solar cells are more promising and low-cost alternative to conventional inorganic photovoltaic devices. TiO 2 nanocrystalline thin films have been sensitized with the natural dye (malvidin- 3-fructoside) extracted from grape fruits. The interaction between the semiconductor and the dye has been studied. The maximum absorption band of grape extract at 525 nm has been shifted to 545 nm after incorporation of the dye indicating interaction of the dye molecules with TiO 2 . The grape extract has polyphenolic anthocyanin pigment (malvidin-3-fructoside), which has carboxylic and hydro- xyl groups that can attach effectively to the surface of TiO 2 film. The dye-sensitized TiO 2 -based solar cell sensitized using grape dye exhibited a J sc of 4.06 mA/cm 2 , V oc of 0.43 V, FF of 0.33 and efficiency of 0.55 %. Natural dye- sensitized TiO 2 photo electrodes present the prospect to be used as an environment-friendly, low-cost alternative system. Keywords Nanocrystalline Sol–gel TiO 2 thin films Dye-sensitized solar cell Introduction Nanocrystalline TiO 2 films have attracted scientific and technological interests because of their potential applica- tions in the fields of photo electronic optical devices, solar cells, gas sensors, photocatalysts and biomaterials [13]. TiO 2 is the best photo-catalyst so far, that absorbs the light at shorter wavelength below 400 nm, due to its wide band gap. Dye-sensitized solar cell, a device converting light energy to electrical energy by imitating photosynthesis of plants [4], was firstly developed by Gratzel’s group [57]. It is widely known as a low-cost and easy-assembly solar cell, in which both synthetic and natural dyes can be used as a sensitizer. Electrical energy is generated when the cell is exposed to sunlight. Electrons in dye molecules are excited and then injected to the conduction band of a wide band gap (TiO 2 ) n-type semiconductor on which the dye molecules adsorb. These electrons migrate through the host semiconductor particles until they reach the collector; also the holes simultaneously generated are reduced by a redox electrolyte or hole carrier at the back electrode (Pt). Because of low material cost, simple preparation, and lack of heavy metals, natural dyes are more favorable to be applied in such device [812]. Solar cells are the basic block of solar array where the absorption of light quanta of specific energy results in generation of charge carriers. Due to absorption of sun light, dye molecules get excited from the highest occupied molecular orbital’s (HOMO) to the lowest unoccupied molecular orbital’s (LUMO). D þ hv ! D ð1Þ Once an electron injected into the conduction band of the wide band gap semiconductor nano structured TiO 2 film, the dye molecule (photosensitizer) become oxidized. N. Gokilamani (&) N. Muthukumarasamy A. Ranjitha Department of Physics, Coimbatore Institute of Technology, Coimbatore, India e-mail: [email protected] M. Thambidurai Department of Electrical and Computer Engineering, Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 151-744, Republic of Korea D. Velauthapillai Department of Engineering, University College of Bergen, Bergen, Norway 123 Mater Renew Sustain Energy (2014) 3:33 DOI 10.1007/s40243-014-0033-6

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Page 1: Grape pigment (malvidin-3-fructoside) as natural sensitizer …...Grape pigment (malvidin-3-fructoside) as natural sensitizer for dye-sensitized solar cells N. Gokilamani • N. Muthukumarasamy

ORIGINAL PAPER

Grape pigment (malvidin-3-fructoside) as natural sensitizerfor dye-sensitized solar cells

N. Gokilamani • N. Muthukumarasamy •

M. Thambidurai • A. Ranjitha •

Dhayalan Velauthapillai

Received: 18 January 2014 / Accepted: 24 May 2014 / Published online: 6 July 2014

� The Author(s) 2014. This article is published with open access at Springerlink.com

Abstract TiO2 nanocrystalline thin films have various

applications, among them dye-sensitized solar cells are

more promising and low-cost alternative to conventional

inorganic photovoltaic devices. TiO2 nanocrystalline thin

films have been sensitized with the natural dye (malvidin-

3-fructoside) extracted from grape fruits. The interaction

between the semiconductor and the dye has been studied.

The maximum absorption band of grape extract at 525 nm

has been shifted to 545 nm after incorporation of the dye

indicating interaction of the dye molecules with TiO2. The

grape extract has polyphenolic anthocyanin pigment

(malvidin-3-fructoside), which has carboxylic and hydro-

xyl groups that can attach effectively to the surface of TiO2

film. The dye-sensitized TiO2-based solar cell sensitized

using grape dye exhibited a Jsc of 4.06 mA/cm2, Voc of

0.43 V, FF of 0.33 and efficiency of 0.55 %. Natural dye-

sensitized TiO2 photo electrodes present the prospect to be

used as an environment-friendly, low-cost alternative

system.

Keywords Nanocrystalline � Sol–gel � TiO2 thin films �Dye-sensitized solar cell

Introduction

Nanocrystalline TiO2 films have attracted scientific and

technological interests because of their potential applica-

tions in the fields of photo electronic optical devices, solar

cells, gas sensors, photocatalysts and biomaterials [1–3].

TiO2 is the best photo-catalyst so far, that absorbs the light

at shorter wavelength below 400 nm, due to its wide band

gap. Dye-sensitized solar cell, a device converting light

energy to electrical energy by imitating photosynthesis of

plants [4], was firstly developed by Gratzel’s group [5–7].

It is widely known as a low-cost and easy-assembly solar

cell, in which both synthetic and natural dyes can be used

as a sensitizer. Electrical energy is generated when the cell

is exposed to sunlight. Electrons in dye molecules are

excited and then injected to the conduction band of a wide

band gap (TiO2) n-type semiconductor on which the dye

molecules adsorb. These electrons migrate through the host

semiconductor particles until they reach the collector; also

the holes simultaneously generated are reduced by a redox

electrolyte or hole carrier at the back electrode (Pt).

Because of low material cost, simple preparation, and lack

of heavy metals, natural dyes are more favorable to be

applied in such device [8–12].

Solar cells are the basic block of solar array where the

absorption of light quanta of specific energy results in

generation of charge carriers. Due to absorption of sun

light, dye molecules get excited from the highest occupied

molecular orbital’s (HOMO) to the lowest unoccupied

molecular orbital’s (LUMO).

D þ hv ! D� ð1Þ

Once an electron injected into the conduction band of

the wide band gap semiconductor nano structured TiO2

film, the dye molecule (photosensitizer) become oxidized.

N. Gokilamani (&) � N. Muthukumarasamy � A. Ranjitha

Department of Physics, Coimbatore Institute of Technology,

Coimbatore, India

e-mail: [email protected]

M. Thambidurai

Department of Electrical and Computer Engineering, Global

Frontier Center for Multiscale Energy Systems, Seoul National

University, Seoul 151-744, Republic of Korea

D. Velauthapillai

Department of Engineering, University College of Bergen,

Bergen, Norway

123

Mater Renew Sustain Energy (2014) 3:33

DOI 10.1007/s40243-014-0033-6

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D� þ TiO2 ! Dþ þ e� TiO2ð Þ ð2Þ

The injected electron is transported between the TiO2

nanoparticles and then gets extracted to a load where the

work done is delivered as an electric energy.

e� TiO2ð Þ þ Electrode ! TiO2 þ e� Electrodeð Þ þ Energy

ð3Þ

To mediate electron between the TiO2 photoelectrode

and the platinum counter electrode, electrolyte containing

I-/I�3 redox ions is used to fill the cell. Therefore, the

oxidized dye molecules (photosensitizer) are regenerated

by receiving electrons from the I- ion redox mediator that

will oxidized to I�3 (Tri-iodide ions).

Dþ þ e� ! D þ I�3 ð4Þ

The I�3 substitutes the donated electron internally with

that from the external load and gets reduced back to I- ion.

2e� Electrodeð Þ þ I�3 ! 3I� þ Electrode ð5Þ

Therefore, generation of electric power in dye-sensitized

solar cell causes no permanent chemical change or

transformation. The conducting electrodes are prepared

such that they posses low-sheet resistance and very high

transparency in order to facilitate high solar cell

performance [13]. Figure 1 shows the structure of a dye-

sensitized solar cell.

Anthocyanins consists a large family of widespread

flavonoids in plants and they are responsible for many fruit

and floral colors that are observed in nature. Chemically,

these flavonoids are most commonly based on six antho-

cyanidins: pelargonidin, cyanidin, peonidin, delphinidin,

petunidin and malvidin [14]. Interest in the anthocyanins

has increased because of their potential as natural colorants

and non-toxicity. They have bright attractive colors, and

they are water-soluble. Acylated anthocyanin pigments

show greater stability during processing and storage.

Because of the presence of acylated groups in the structure

of anthocyanins, it is believed that they can protect the

oxonium ion from hydration, thereby prevent the formation

of hemiketal (pseudobase) or chalcone forms. Anthocyanin

pigments were found in 1835 for the first time. In 1916,

Wehldale proposed anthocyanin flavanons formation

pathway, since they were products of shikimic acid cycle.

Willstatter and Zollinger [15, 16] reported malvidin

3-fructoside as the major pigment in grapes, and malvidin

3,5-diglucoside and malvidin as the minor ones. Antho-

cyanins from grapes include mono and diglucosides of five

different aglycones with the addition of monoacylation

[14]. The structure of malvidin 3-fructoside is shown in

Fig. 2.

Sol–gel dip coating method has been used because of the

low-cost equipments involved, lower growth temperatures

and reproducibility. Many sensitizers have been used by

different workers to sensitize TiO2 nanoparticles for solar

cell applications. In this paper, we report a simple route to

synthesize TiO2 nanoparticles. TiO2 nanoparticles have

been used as photoelectrode material to fabricate the dye-

sensitized solar cells. The use of non-toxic natural pig-

ments as sensitizers would definitely enhance the envi-

ronmental and economic benefits of this alternative form of

solar energy conversion. Natural dye-sensitized solar cells

are a promising class of photovoltaic cells with the capa-

bility of generating green energy at low production cost

since no vacuum systems or expensive equipment are

required in their fabrication. In addition, natural dyes are

abundant, easily extracted and safe materials.

Experimental

To prepare the photo-anode of dye-sensitized solar cells,

the indium-doped tin oxide conducting (ITO) glass sheet

Fig. 1 Schematic diagram of dye-sensitized solar cell

Fig. 2 Structure of malvidin-3-fructoside

Page 2 of 7 Mater Renew Sustain Energy (2014) 3:33

123

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(Asahi Glass; Indium-doped SnO2, sheet resistance: 15 U/

square) was first cleaned in a detergent solution using an

ultrasonic bath for 15 min, rinsed with double distilled

water and then dried. The matrix sol was prepared by

mixing 0.695 ml of titanium (IV) isopropoxide with 15 ml

of isoproponal at room temperature and stirred for half an

hour. 0.135 ml of glacial acetic acid is added drop wise and

stirred vigorously for 2 h to obtain a homogeneous mixture

of TiO2 sol. The prepared sol was deposited on the Indium-

doped tin oxide (ITO) glass plate by sol–gel dip coating

method. The film was dried at 80 �C for 30 min in air and

then annealed at 500 �C in a muffle furnace. For grapes a

dye extract preparation, well cleaned grape fruits were

mixed with 250 ml ethanol and were kept for 12 h at room

temperature. Then residual parts were removed by filtra-

tion. The ethanol fraction was separated and few drops of

concentrated HCl was added so that the solution became

deep red color (pH \ 1). This was directly used as dye

solution for sensitizing TiO2 electrodes.

Crystallinity and phase analysis of the films were carried

out using X-ray diffraction method (Rigaku Rint 2000

series). Energy dispersive X-ray analysis (EDAX) and

structural studies have been carried out using transmission

electron microscope (JEOL, JEM-2100). Transmission

spectra have been recorded using UV–VIS–NIR spectro-

photometer (Jasco V-570). Lithium iodide, iodine and

acetonitrile purchased from Sigma-Aldrich have been used

as received for the preparation of electrolyte. The redox

electrolyte with [I�3 ]/[I-] 1:9 was prepared by dissolving

0.5 M LiI and 0.05 M I2 in acetonitrile solvent. Since LiI is

extremely hygroscopic, electrolytes were prepared in a dry

room maintained at dew point of 60 �C. The counter

electrode was prepared by using platinum chloride as fol-

lows: the H2PtCl6 solution in isopropanol (2 mg/ml) was

deposited onto the ITO glass by spin coating method. TiO2

electrode annealed at 500 �C was immersed in the

extracted dye solution at room temperature for 24 h in the

dark, since the grape dye extract is not stable under pro-

longed illumination of light. The electrode was then rinsed

with ethanol to remove the excess dye present in the

electrode and then the electrode was dried. The counter

electrode was placed on the top of the TiO2 electrode, such

that the conductive side of the counter electrode facing the

TiO2 film with a spacer separating the two electrodes. The

two electrodes were clamped firmly together using a binder

clip. Now the prepared liquid electrolyte solution was

injected into the space between the clamped electrodes.

The electrolyte enters into the cell by capillary action. This

resulted in the formation of sandwich type cell. Natural

dye-sensitized TiO2-based solar cells have been fabricated

with area of 0.25 cm2, and it was found that the cell effi-

ciency was independent of cell area in this range as

reported by Yamazaki et al. [17]. The J–V characteristics

of the cell were recorded using a Keithley 4200-SCS meter.

A xenon lamp source (Oriel, USA) with an irradiance of

100 mW/cm2 was used to illuminate the solar cell

(equivalent to AM1.5 irradiation).

Results and discussion

Figure 3 shows the X-ray diffraction pattern of the sol–gel

prepared TiO2 films, annealed at 500 �C. A narrow peak at

25.35� corresponding to (101) reflection of the anatase

phase of TiO2 has been observed in the diffraction pattern.

The observed peaks corresponds to (1 0 1), (0 0 4), (2 0 0),

(1 0 5), (2 1 1) and (2 0 4) which represent only anatase

phase of TiO2. No peaks corresponding to the rutile or

brookite phase has been observed. The grain size has been

calculated using Scherer’s formula [18].

D ¼ Kk=bcosh ð6Þ

where D is the grain size, K is a constant taken to be 0.94, kis the wavelength of the X-ray radiation (k = 1.51 A

´), b is

the full width at half maximum and h is the angle of dif-

fraction. The grain size was found to be 33 nm for the film

annealed at 500 �C. The annealing temperature facilitates

the subsequent crystal growth process, accompanied by the

diffusion of titania species forming big size anatase crystals

formed by merging of some adjacent mesopores. At the

same time, the spatial confinement by mesopore arrays

controls the formation and growth of anatase phase, leading

to a more or less uniform distribution of titania

nanocrystals.

Energy dispersive X-ray (EDAX) spectra of nanocrys-

talline TiO2 thin film is shown in Fig. 4. The compositional

analysis of TiO2 thin films shows the presence of 32.7 at%

10 20 30 40 50 60 70 80

(204

)

(105

)

(200

)

(004

)

(101

)

Inte

nsi

ty (

a.u

.)

2θ (degrees)

Fig. 3 X-ray diffraction pattern of annealed at 500 �C TiO2 thin

films

Mater Renew Sustain Energy (2014) 3:33 Page 3 of 7

123

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of Ti and 67.3 at% of O. Figure 5 shows the transmission

electron microscope images of the TiO2 thin film annealed

at 500� C. Figure 5a shows the presence of close-packed

agglomerated nanoparticles of uniform size which causes

the mesoporous structure. This accumulation of nanopar-

ticles creates narrow channels that may serve as electronic

injection membranes [19]. The mesoporous structure,

which provides a large surface area for adsorbing the dye,

has been achieved in the present study, although no poly-

mer was added to make pores. The prepared TiO2 films

exhibited good mechanical strength, had good adherence to

the substrate and could not be easily erased by hand. Fig-

ure 5b shows the high-resolution transmission electron

microscope (HRTEM) image of TiO2 thin films. The

interplanar distance has been calculated using the lattice

fringes and is found to be 0.33 nm which corresponds to

(101) lattice plane of anatase phase. Selective area electron

diffraction pattern is used to learn about the crystal prop-

erties of a particular region. Figure 5c shows the selective

area electron diffraction pattern of the nanocrystalline TiO2

thin film and the presence of rings with discrete spots

suggest that the TiO2 nanocrystalline film is made of small

particles of uniform size with anatase phase.

Figure 6a–c shows atomic force microscope images of

the TiO2 thin films annealed 400, 450 and 500 �C. The

image shows well-defined particle-like features with

granular topography and indicates the presence of small

crystalline grains. Because of the heat treatment, the nano

crystalline phase has been formed and this has led to the

appearance of grains making the films to have higher sur-

face roughness. The root mean square surface roughness of

the film was found to be 19, 27 and 32 nm for the TiO2

films annealed at 400, 450 and 500 �C.

To discriminate the local order characteristics of the

TiO2 films, we carried out non-resonant Raman

spectroscopy studies. The technique is non-destructive,

capable to elucidate the titania structural complexity as

peaks from each crystalline phase is clearly separated in

frequency, and therefore the anatase and rutile phases are

easily distinguishable. Figure 7 shows the Raman spectra

of the nanocrystalline TiO2 sample annealed at 500 �C.

Raman peaks originate from the vibration of molecular

Fig. 4 The EDAX spectra of TiO2 thin film

Fig. 5 a TEM image b HRTEM image and c SAED pattern of TiO2

nanocrytalline thin film annealed at 500 �C

Page 4 of 7 Mater Renew Sustain Energy (2014) 3:33

123

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bonds, that is, vibrational mode Eg and A1g peaks, which

are related to different crystal planes. According to factor

group analysis, anatase has six Raman active modes

(A1g ? 2B1g ? 3Eg). Ohsaka et al. [20] have reported the

Raman spectra of an anatase TiO2 and have stated that six

allowed modes appear at 144 cm-1 (Eg), 197 cm-1 (Eg),

399 cm-1 (B1g), 513 cm-1 (A1g), 519 cm-1 (B1g), and

639 cm-1 (Eg). The vibrational peaks of the nanocrystal-

line TiO2 samples at 143, 197, 396, 519, 638 cm-1 and the

absence of overlapped broad peaks show that the material

is well crystallized, with low number of imperfect sites.

These peaks are unambiguously attributed to the anatase

modification. A special attention must be given to the

Raman peaks observed at 143, 197, 396, 519, 638 cm-1

which are all slightly shifted, probably due to a smaller size

of TiO2 nanocrystalline particles. The peak at 513 and

517 cm-1 are very close to each other. The main spectral

features of samples annealed at different temperatures are

closely similar which mean that the prepared samples

posses a certain degree of long range order of the anatase

phase. The spectra vary symmetrically with grain size. It

deteriorates with spectroscopic lines broadening and

merging, line intensity decreases and position shifting with

decrease in annealing temperature. However, the spectrum

of sample showing the band broadening with decrease in

intensity [21].

Optical absorption spectra of grape fruits and grape

fruits absorbed TiO2 nanocrystalline thin films are shown

in Fig. 8. It is seen that the absorption peak of grape fruits

extract is maximum at 510 nm. The presence of the

absorbance peak at 530 nm in grape fruits sensitized TiO2

nanocrystalline thin films confirms the incorporation of dye

into the TiO2 film. Malvidin-3-fructoside flavanoid is

responsible for the absorption of light. This chemical

attachment affects the energy levels of the highest occupied

molecular level (HOMO) and the lowest unoccupied

Fig. 6 AFM images of TiO2 thin films annealed at a 400 �C, b 450 �C and c 500 �C

0 200 400 600 800 1000

(Eg)

Inte

nsi

ty (

a.u

.)

Raman Shift (cm-1)

(Eg)

(B1g) (Eg)

(A1g +B1g)

Fig. 7 The Raman spectra of TiO2 thin film annealed at 500 �C450 475 500 525 550 575 600 625 650

0.2

0.4

0.6

0.8

1.0

Ab

sorb

ance

(a.

u)

Wavelength (nm)

grapes dyegrape +TiO2

Fig. 8 Absorption spectra of grapes sensitized TiO2 thin films

Mater Renew Sustain Energy (2014) 3:33 Page 5 of 7

123

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molecular level (LUMO) of the anthocyanidin molecule

[22], which eventually affects the band gap of these

materials and this results in a shift in the absorption peak of

the absorption spectra. It is seen that absorption coefficients

of grape fruits extract is about 14 times higher than that of

the N-719 dye used in high efficiency dye-sensitized solar

cells [23]. The absorption bands of the dye adsorbed TiO2

semiconductor films are shifted to longer wavelength when

compared to the absorption spectra of the dye solution as

shown in Fig. 8. The intensity of light absorption has been

enhanced due to the interfacial Ti–O coupling which exists

between the C=O, C–H, C–O, O–H bonding of dye mol-

ecules and TiO2 molecules [24].

Figure 9a, b shows the Fourier transform infrared

spectra (FTIR) of grape extract, grape extract-sensitized

TiO2 nanocrystalline thin films. The spectra have been

recorded using FTIR 1600 infrared spectrophotometer,

operating in the wave number range 800–4,400 cm-1. For

the grape extract, the bands in the region of 1,651 cm-1

represents the amide group and the bands in the regions

2,900, 2,978 cm-1 represents the C–H bonding of the

grape extract and a broad sharp peak is obtained in the

region 3,371 cm-1 which corresponds to the hydrogen

bonding. The bands in the region 1,381 cm-1 correspond

to OH bonding. The bands in the region 1,273 and

1,087 cm-1 correspond to O–C bonding and the band at

879 cm-1 represents the C–H bonding in the grape extract.

The grape extract-sensitized TiO2 nanocrystalline thin

films shows a peak at 1,442 cm-1 which corresponds to the

absorption band of TiO2 molecules [25]. The bands ranging

from 2,854 to 3,363 cm-1 correspond to the C–H stretch-

ing due to methyl and methylene groups.

The J–V characteristics of TiO2 nanocrystalline thin

films sensitized with natural dyes is shown in Fig. 10. The

overall efficiency g of the grape dye cells are evaluated in

terms of open circuit voltage (Voc), short circuit current

(Jsc) and fill factor (FF) using the formula

FF ¼ Jmax � Vmax

Jsc � Voc

ð7Þ

g ¼ Jsc � Voc � FF

Pin

ð8Þ

where Jsc is the short circuit photocurrent density

(mA cm-2), Voc the open circuit voltage (volts), Pin is the

intensity of the incident light (Pin = 1 Wcm-2) and Jmax

(mA cm-2) and Vmax (volts) are the maximum current

density and voltage in the J–V curve, respectively, at point

of maximum power output. The solar cell sensitized with

grape extract dye exhibited a power conversion efficiency

of 0.55 % with a short circuit current density (Jsc) of

4.06 mA/cm2, open circuit voltage (Voc) of 0.43 V and fill

factor (FF) of 0.33.

Conclusion

TiO2 nanocrystalline thin films have been prepared by a

simple sol–gel method. X-ray diffraction analysis reveals

that the TiO2 nanocrystalline thin films exhibit anatase

structure. The dyes extracted from grapes strongly absorb

visible light at 510 nm and the absorption bands of the dye

adsorbed TiO2 semiconductor films are shifted to longer

wavelength and have been found to be suitable for the use

as sensitizer in solar cells. The efficiency of the fabricated

dye-sensitized solar cell using grapes extract is 0.55 %.

Unlike artificial dyes, the natural ones are available, easy to

prepare, low in cost, non-toxic, environmentally friendly

and fully biodegradable.

1000 1500 2000 2500 3000 3500 400086

88

90

92

94

96

98

100

102

3363

(b)

Tra

nsm

itta

nce

(%

)

Wavenumber (cm )-1

(a)

3371

2978

2900

1730

1622

1442

1381

1273

1087

1057

879

1087

2306

1651

2854

Fig. 9 FTIR spectra of a grape extract, b grape extract-sensitized

TiO2

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5

-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0.001

Cu

rren

t d

ensi

ty (

A/c

m2)

Voltage(V)

Voc =0.43 V Jsc =4.06 mA/cm2

FF = 0.33 = 0.50%η

Fig. 10 J–V characteristics of dye-sensitized TiO2-based solar cells

Page 6 of 7 Mater Renew Sustain Energy (2014) 3:33

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Open Access This article is distributed under the terms of the

Creative Commons Attribution License which permits any use, dis-

tribution, and reproduction in any medium, provided the original

author(s) and the source are credited.

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